Many different oxygen separation systems, for example, organic polymer membrane systems, have been used to separate selected gases from air and other gas mixtures. Air is a mixture of gases which may contain varying amounts of water vapor and, at sea level, has the following approximate composition by volume: oxygen (20.9%), nitrogen (78%), argon (0.94%), with the balance consisting of other trace gases. An entirely different type of membrane, however, can be made from certain inorganic oxides. These solid electrolyte membranes are made from inorganic oxides, typified by calcium- or yttrium-stabilized zirconium and analogous oxides having a fluorite or perovskite structure.
Some of these solid oxides have the ability to conduct oxygen ions at elevated temperatures if an electric potential is applied across the membrane, that is, they are electrically-driven or ionic conductors only. Recent research has led to the development of solid oxides which have the ability to conduct oxygen ions at elevated temperatures if a chemical driving potential is applied. These pressure-driven ionic conductors or mixed conductors may be used as membranes for the extraction of oxygen from oxygen-containing gas streams if a sufficient partial oxygen pressure ratio is applied to provide the chemical driving potential. Since the selectivity of these materials for oxygen is infinite and oxygen fluxes generally several orders of magnitude higher than for conventional membranes can be obtained, attractive opportunities are created for the production of oxygen using these ion transport membranes.
Although the potential for these oxide ceramic materials as gas separation membranes is great, there are certain problems in their use. The most obvious difficulty is that all of the known oxide ceramic materials exhibit appreciable oxygen ion conductivity only at elevated temperatures. They usually must be operated well above 500.degree. C., generally in the 600.degree. C.-900.degree. C. range. This limitation remains despite much research to find materials that work well at lower temperatures. Solid electrolyte ionic conductor technology is described in more detail in Prasad et al., U.S. Pat. No. 5,547,494, entitled Staged Electrolyte Membrane, which is hereby incorporated by reference to more fully describe the state of the art.
Combustion processes, however, usually operate at high temperature and therefore there is the potential for efficiently integrating ion transport systems with oxygen enhanced combustion processes and the present invention involves novel schemes for the integration of ion transport systems with oxygen enhanced combustion processes.
Most conventional combustion processes use the most convenient and abundant source of oxygen, that is, air. The presence of nitrogen in air does not benefit the combustion process and, on the contrary, may create many problems. For example, nitrogen reacts with oxygen at combustion temperatures, forming nitrogen oxides (NO.sub.x), an undesirable pollutant. In many instances, the products of combustion must be treated to reduce nitrogen oxide emissions below environmentally acceptable limits. Moreover, the presence of nitrogen increases the flue gas volume which in turn increases the heat losses in the flue gas and decreases the thermal efficiency of the combustion process. To minimize these problems, oxygen-enriched combustion (OEC) has been commercially practiced for many years. There are several benefits of oxygen-enriched combustion including reduced emissions (particularly nitrogen oxides), increased energy efficiency, reduced flue gas volume, cleaner and more stable combustion, and the potential for increased thermodynamic efficiency in downstream cycles. These benefits of OEC, however, must be weighed against the cost of the oxygen that has to be manufactured for this application. As a consequence, the market for OEC is greatly dependent on the cost of producing oxygen-enriched gas. It has been estimated that as much as 100,000 tons per day of oxygen would be required for the new markets in OEC if the cost of oxygen-enriched gas could be reduced to about $15/ton. It appears that gas separation processes employing ion transport membranes have the promise of reaching that goal. OEC is discussed in detail in H. Kobayashi, Oxygen Enriched Combustion System Performance Study, Vol. 1: Technical and Economic Analysis (Report #DOE/ID/12597), 1986, and Vol. 2: Market Assessment (Report #DOE/ID/12597-3), 1987, Union Carbide Company-Linde Division, Reports for the U.S. Dept. of Energy, Washington, D.C.).
Literature related to ion transport conductor technology for use in separating oxygen from a gas stream includes:
Hegarty, U.S. Pat. No. 4,545,787, entitled Process for Producing By-Product Oxygen from Turbine Power Generation, relates to a method of generating power from a compressed and heated air stream by removing oxygen from the air stream, combusting a portion of the resultant air stream with a fuel stream, combining the combustion effluent with another portion of the resultant air stream, and expanding the final combustion product through a turbine to generate power. Hegarty mentions the use of silver composite membranes and composite metal oxide solid electrolyte membranes for removing oxygen from the air stream.
Kang et al., U.S. Pat. No. 5,516,359, entitled Integrated High Temperature Method for Oxygen Production, relates to a process for separating oxygen from heated and compressed air using a solid electrolyte ionic conductor membrane where the nonpermeate product is heated further and passed through a turbine for power generation.
Mazanec et al., U.S. Pat. No. 5,160,713, entitled, Process for Separating Oxygen from an Oxygen-Containing Gas by Using a Bi-containing Mixed Metal Oxide Membrane, discloses bismuth-containing materials that can be used as oxygen ion conductors.
Publications related to oxygen-enriched or enhanced combustion (OEC) include the above-mentioned U.S. Dept. of Energy reports authored by H. Kobayashi and H. Kobayashi, J. G. Boyle, J. G. Keller, J. B. Patton and R. C. Jain, Technical and Economic Evaluation of Oxygen Enriched Combustion Systems for Industrial Furnace Applications, in Proceedings of the 1986 Symposium on Industrial Combustion Technologies, Chicago, Ill., Apr. 29-30, 1986, ed. M. A. Lukasiewics, American Society for Metals, Metals Park, Ohio, which discusses the various technical and economic aspects of oxygen-enhanced combustion systems.
Oxygen-enriched combustion has been commercially practiced using oxygen manufactured by either cryogenic distillation or noncryogenic processes such as pressure swing adsorption (PSA). All of these processes operate at or below 100.degree. C. and therefore are difficult to thermally integrate with combustion processes.
When the boiler of a steam power plant is fired with oxygen and fuel, the power required to separate air in the state-of-the-art cryogenic plant is very significant and consumes about 16% of the total power generated from the single cycle steam boiler power plant. The compression of air required for air separation is the primary source of this power requirement.
Oxygen is too expensive to use for most boiler applications. In a typical air-fuel fired boiler operation, air is fed at a pressure of several inches of H.sub.2 O into the combustion chamber that operates at about the atmospheric pressure. Compressing air to a low pressure of even a few psig is considered too costly due to the increased power requirement for compression and a consequent loss of power generation efficiency.
One practical problem of using ceramic membranes is the lack of control resulting from leakage at ceramic joints and through cracks in the ceramic membrane tubes. Ceramic materials are susceptible in developing stress cracks when used at high temperatures, and especially under changing temperature conditions. Therefore, it is highly desirable to develop a robust ceramic membrane system that can continue to operate efficiently and effectively despite ceramic membrane tube cracks due to thermal and mechanical stress.